The present invention relates in general to circuits for protecting telephone lines and equipment from overvoltage and overcurrent conditions, and more particularly to an integrated circuit that accomplishes both such functions.
Many telephone lines are situated so as to be subjected to external electrical interferences such as power line crosses, lightning strikes, electromagnetic fields, etc. Many different types of electrical circuits and apparatus have been designed to protect telephone lines from overvoltage and overcurrent conditions resulting from such disturbances. The applicable protection circuits are generally incorporated into line interface boards and other circuit modules that are utilized to terminate and otherwise interface the telephone lines to central office or subscriber circuits.
Twisted pair telephone lines typically carry up to 150 milliamp of loop current during normal operating conditions. Overcurrent conditions of a telephone line can be experienced when sneak currents in excess of the loop currents are coupled to the telephone lines. Sneak currents may constitute those currents which are not related to telephone signaling or information carried on the lines, but rather those stray AC and DC currents which contribute no useful information. Overcurrent conditions may also constitute those catastrophic currents carried as a result of lightning strikes or power line crosses.
As noted above, various types of apparatus have been devised for protecting the telephone lines and associated circuits from such overcurrent conditions. Mechanical heat coils are well known in the art for sensing overcurrent conditions. When an excessive current is carried by the telephone line, such current is carried through a heat coil which generates thermal energy sufficient to melt a soldered connection. Once the soldered connection melts, a spring loaded mechanism operates to thereby short circuit one or both conductors of the telephone line to ground. Many different types of heat coils are employed, but all such types of equipment rely on the melting point of solder, which is generally between 140-160xc2x0 F. Many conventionally available heat coils are inappropriate for use in outdoor cabinets and other non-ventilated structures where the ambient temperature can rise above the melting point of the solder. When this occurs, the heat coils can be inadvertently activated due to the ambient heat alone. Once activated, the heat coils do not automatically reset, but must be manually replaced.
Overcurrent conditions can also be sensed by positive temperature coefficient (PTC) elements, often referred to as thermistors, placed in series with the telephone line. PTC elements are generally constructed of ceramic and other types of materials which increase in resistance as a function of the body temperature thereof which, in turn is a function of the current carried therethrough. Accordingly, as the telephone line current increases above an unsafe condition, the resistance of the PTC element increases, thereby restricting the flow of line current. In this instance, the telephone line is not shunted by the current limiting device, as is typical with heat coils, but rather the effective line resistance increases to prevent the subscriber equipment from being subjected to the overcurrent condition. An advantage of the use of PTC elements is that once the overcurrent condition is removed, the resistance of the device decreases to its steady state value, such as 8-10 ohms. The disadvantage with PTC elements is that they are expensive, bulky and difficult to scale down in size.
Yet another device employed to provide overcurrent protection to telephone lines are simple fuses. Series fuses in telephone lines are well known and readily used, but nevertheless are somewhat expensive due to the difficulty in manufacturing low-current fuses. Excessive currents in telephone lines may constitute 250 milliamps or more. The ability to accurately manufacture low amperage fuses is not easily carried out, thus the screening and manufacturing measures result in the increased cost of such fuses. Much like heat coils, once a fuse is blown, it must be replaced, and until replaced the telephone line cannot be used to carry information. Moreover, when heat is the mechanism that activates the protection device, it is generally slow reacting, thereby allowing an initial overcurrent to be passed to the circuits to be protected.
Overvoltage protection circuits are also well known in the field for protecting telephone lines from overvoltages, such as caused by lightning strikes and power line crosses. A host of solid state devices are readily available and, when placed in parallel with either the tip and ring conductors, or both, cause such lines to be either short circuited together, or short circuited to ground in response to the overvoltage condition. Various solid state overvoltage devices have been highly developed for providing precise overvoltage protection to telephone lines. Indeed, many different types of solid state overvoltage protection devices are available from Teccor Electronics, Irving, Tex., under the brand SIDACtor(copyright). Such devices are characterized by a predefined breakover voltage, low initial voltage overshoot and a high current carrying capability. Many other thyristor, transistor and diode arrangements and combinations of devices, including gas discharge tube, carbon block air gaps have been utilized to provide overvoltage protection to telephone lines.
Telephone line protection modules and circuits generally require at least overvoltage protection capabilities. Yet other, more sophisticated protection applications require both overvoltage and overcurrent protection capabilities. However, the provision of both overvoltage and overcurrent protection generally involves the use of separate and distinct devices and circuits within the protection module to carry out the different functions. Stated another way, there is generally not available a single semiconductor device that provides both overcurrent and overvoltage capabilities.
From the foregoing, it can be seen that a need exists for a single device that provides both overcurrent and overvoltage protection capabilities. Yet another need exists for a semiconductor device that provides both overvoltage and overcurrent protection functions in a high speed manner, and is not destroyed once activated. Another need exists for a low cost device that provides both overvoltage and overcurrent protection capabilities, and which automatically resets so that replacement or manual resetting is not necessary. A need exists for a single low cost semiconductor device that provides both overcurrent and overvoltage protection without accompanying circuits or components.
In accordance with the principles and concepts of the present invention, there is provided a solid state protection device that overcomes and substantially reduces the shortcomings of the corresponding prior art circuits, modules and devices. In accordance with a preferred form of the invention, a single integrated circuit provides both overcurrent and overvoltage protection capabilities in the same chip. This not only simplifies the fabrication and assembly of electrical protection modules, but also reduces the cost thereof No solder melting techniques or moving parts are required to provide either the overcurrent or overvoltage protection capabilities. Moreover, the device according to the invention reacts in a high speed manner to both overvoltage and overcurrent conditions without being destroyed or otherwise having to be manually reset.
In accordance with the preferred embodiment of the invention, a bidirectional current carrying device, such as a specially designed triac, is utilized to provide overcurrent and overvoltage protection to the telephone line ring conductor circuits. In like manner, a specially designed triac is employed to provide overcurrent and overvoltage protection to the telephone line tip conductor circuits. Two-terminal SIDACtor(copyright) overvoltage devices produced by Teccor Electronics are formed in a single semiconductor chip, with multiple buried regions, shorting dots, and the like, to provide a precise breakover voltage in response to an overvoltage condition, to operate with only a small initial voltage overshoot, as well as provide a high current carrying capability. Such devices are described in U.S. Pat. No. 5,479,031 by Webb et al. However, rather than utilizing the standard two-terminal (anode/cathode) device, the present semiconductor device is fabricated to additionally provide a gate terminal that is externally accessible, much like a triac. When used in telephony applications, there is formed between the gate terminal and the cathode of the semiconductor device a resistance of less than about 20 ohms. This resistance is formed by appropriately doping the semiconductor material so that when a current of about 180-300 milliamp passes therethrough, a cathode-gate junction threshold voltage is reached, in which event the device is driven to its on-state condition. Hence, any telephone line overcurrent condition of about 180-300 milliamp causes the modified device to turn on, independent of any overvoltage condition. Two similar devices are formed in a single semiconductor chip to thereby provide both overcurrent and overvoltage protection to both telephone line conductors.
In the fabrication of the semiconductor devices according to the invention, an edge gate design is utilized. The semiconductor regions are formed with various geometries to accomplish balanced four quadrant operation.
Optionally, the telephone line protection device may not only include the single overcurrent and overvoltage chip, but may also utilize a mechanical failsafe switch to short circuit the tip and ring conductors to ground in case of a sustained overvoltage condition on the telephone line. This protects the semiconductor chip, as well as the subscriber circuits from sustained overvoltage conditions which might otherwise damage such equipment.
In one embodiment of the invention, there is included a semiconductor device providing overvoltage and overcurrent protection to a conductor, comprising a cathode, anode and gate terminal, a plurality of semiconductor regions of said device arranged to provide overvoltage protection to said conductor when an overvoltage is impressed between said cathode and anode terminals, irrespective of a magnitude of a gate-cathode current, and a plurality of semiconductor regions of said device arranged to provide overcurrent protection to said conductor when a current on the conductor exceeding a specified threshold passes through said gate and cathode terminals, and when said specified threshold is exceeded, the device is driven into a conduction state in which a magnitude of a cathode-anode voltage is low.
In another form of the invention, there is included a semiconductor device for use in protecting telephone line equipment from overvoltage and overcurrent conditions, comprising a first bidirectional current carrying device formed in a semiconductor material, said first bidirectional current carrying device including an anode, a cathode and a gate, and being responsive to an overvoltage of a specified magnitude between the anode and cathode for driving said first bidirectional device into a conduction state, a second bidirectional current carrying device formed in a semiconductor material, said second bidirectional current carrying device including an anode, a cathode and a gate, and operating substantially identical to said first bidirectional current carrying device, a first resistance formed in said semiconductor material across the gate and said cathode of said first bidirectional current carrying device, said first resistance being of a value selected for triggering said first bidirectional current carrying device when an overcurrent of a predefined value flows between a telephone line tip conductor and a customer equipment tip conductor, and a second resistance formed in said semiconductor material across the gate and said cathode of said second bidirectional current carrying device, said second resistance being of a value selected for triggering said second bidirectional current carrying device when an overcurrent of a predefined value flows between a telephone line ring conductor and a customer equipment ring conductor.